For the decaffeination of coffee, a fairly large number of processes have been proposed in the past. Many if not most of these processes have in common a pretreatment of the raw coffee with water, followed by a treatment with solvents designed to act as selectively as possible on the caffeine to dissolve and permit the removal of the same. Prime examples of solvents proposed in the prior art are esters, aromatic hydrocarbons and particularly, halogenated hydrocarbons used either by themselves or as mixtures. Also known is the treatment of aqueous extracts of raw or roasted coffee, in counter-current apparatus, e.g. with chlorinated hydrocarbons whereby to remove the caffeine, the aqueous extract free from caffeine being returned to the extracted beans.
The solvent used must be completely removed, at elevated temperature, from the beans and the extracts, a requirement usually met by steaming with water vapor, a procedure which entails technical and analytical complications and expenditure. For this reason, numerous attempts have been made to remove the caffeine from coffee in some other way. Thus, U.S. Pat. No. 1,640,648, for example, proposed a method utilizing the property of caffeine to sublimate at elevated temperatures. In an initial stage of this process, the raw coffee, in order to be liberated from caffeine, is treated with alkaline substances. Subsequently, the raw coffee is heated to 178.degree. C. and while at this temperature, exposed to a current of inert gas passed through it. The caffeine which sublimates at this temperature, is carried away by the gas current. Inert gases proposed include hydrogen, nitrogen and carbon dioxide. This process did not succeed in attaining terminal values below 0.35 percent of caffeine in the raw coffee, with the result that coffee so processed may not be considered to be free from caffeine, or to have a low caffeine content, by the standard set e.g. by German nutritional regulations. Further important drawbacks of this method include exposure of the coffee to alkali, and to high temperatures.
Another process disclosed in (printed German application) DT-AS No. 2,005,293 removes caffeine from moistened raw coffee by means of supercritical, i.e. gaseous carbon dioxide. The preferred range of operation is between 40.degree. C. and 80.degree. C.; the lower limit is the critical temperature of the carbon dioxide. The treatment of the raw coffee at temperatures within this range yields a coffee which when roasted, fails to have a fully satisfying taste.
Further research demonstrated that caffeine may be removed from raw coffee also at temperatures below the critical temperature, by means of liquid carbon dioxide. The decaffeination works with a mixture of liquid and gaseous carbon dioxide as well as with liquid carbon dioxide alone (i.e. at pressures somewhat above the vapor pressure of the liquid carbon dioxide). Under these conditions, however, the dissolving power of the liquid carbon dioxide is not very selective so that in addition to caffeine, other substances are removed from the raw coffee which play an important part in the formation of aroma in the course of roasting. Coffee treated with liquid CO.sub.2, therefore, suffers from an inferior aroma.
The properties of liquid carbon dioxide when used as a solvent for caffeine, have been investigated and described a number of times. Thus M. Sivetz, in "Coffee Processing Technology", vol. 2, pp. 21-23, discusses the recovery of coffee aroma oil. In this connection, British Pat. No. 11 06468 and Austrian Pat. No. 2 85 307 may also be mentioned. Similarly, the production of other aroma concentrates by means of liquid carbon dioxide has been described e.g. in "Food Technology" No. 23,11,50 (1969); such aroma concentrates are mixtures of a very large number of components. Thus, in the extraction of raw coffee with liquid carbon dioxide, removal of caffeine is accompanied by the simultaneous removal of other substances. As a result, the caffeine recovered is badly contaminated while the coffee lacks components which have an important influence on the formation of aroma.
A more recent proposal made in DT-AS No. 22 12 281 utilizes the fact that an increase of pressure in the extractor, greatly increases the selectivity of liquid carbon dioxide for caffeine. With pressures above the critical pressure, in particular, the caffeine removed from the coffee is substantially pure, while the content, in the coffee, of substances important for aroma formation, is not noticeably reduced. This process involves treatment of raw coffee with water until a moisture content of from 10 to 60 percent by weight has been obtained, and selective extraction of caffeine with liquid carbon dioxide saturated with water, at a pressure exceeding the critical pressure.
The caffeine so extracted is recovered as a white powder of a purity above 98 percent. Preferably, the extraction is carried out in the temperature range between 0.degree. C. and the critical temperature of the carbon dioxide, and at pressures above 80 bar. The moisture content of the raw coffee is obtained, in known manner, by a steaming preceeding the extraction. The caffeine dissolved in the liquid CO.sub.2 is removed in an activated carbon adsorber. The activated carbon, which previously had been saturated with water, adsorbs the dissolved caffeine quantitatively. The solvent is purified and recycled by a pump.
Instead of purifying the circulating medium by means of an activated carbon adsorber, the caffeine may be separated from the solvent also by first conducting the caffeine containing solvent into a separator, for evaporation therein. The vapor is condensed in a cooler and returned by a liquid pump to the extractor. On evaporation the solvent, the caffeine is retained quantitatively in the separator for ready discharge therefrom. Inasmuch as in the course of evaporation in the separator, the carbonic acid suffers from a loss of water, a corresponding volume of water must be added to the carbonic acid before it is returned to the extractor.